You are a member of the remnants of a chapter so you can be anything however the chance of everyone being a librarian is very slim, you get your role through a dice role and even if they do, its going to be a long time before they would be over powering and by that point id be making the game a lot harder. its not a problem if your over powered if your W is low and you only take a hit or two.

"I am Honored to call every single one of you my brother.....You will die today, I will die today but if we must die for the greater good of humanity so be it. With our deaths, our children will blossom into the stars with the Primarches of anew and start a new, grander crusade in the name of the Emperor and as the Emperor's chosen, we will guide them to that path......Show them the true fury of the Emperor's wraith one last time my brothers!.....Forward in the name of the Emperor and may you find salvation with the Emperor when the end comes for you.